Craniofrontonasal syndrome (CFNS) is an X-linked disease that affects multiple aspects of craniofacial development and is caused by mutations in the EFNB1 gene. Frontonasal dysplasia is a defining attribute of CFNS, but the developmental etiology underlying ephrin-B1 function in this context is incompletely defined. We propose to utilize the mouse as a model to understand the basic developmental mechanisms by which EFNB1 acts to control development of the midface. CFNS is an unusual disease in that whereas EFNB1 is an X-linked gene, heterozygous females are more severely affected than hemizygous males. This is related to the fact that heterozygous females represent a mosaic of ephrin-B1 expressing and non-expressing cells. Using a combination of mouse genetics and molecular biology approaches, we will define the mechanistic basis for this phenomenon. Finally, we propose a possible general strategy for preventing CFNS, whose feasibility we will begin to test in a mouse model.